Post-tensioning is a type of steel reinforcement for concrete structures. It typically refers to the method of placing wet concrete around unstressed tendons that have been coated with grease and encased in a plastic sheath. After sufficient hardening of the concrete the tendons are pulled, or post-tensioned, to provide strength to the structure. The tendons are then anchored and cut off just inside the face of the structure in what are termed"pockets". The"pockets" surrounding these tendon ends are then filled with a concrete grout.
The basic procedures used in prestressing or post-tensioning concrete structures have been known since the 1940's, and development of these procedures is traceable to the late 1920's. However, general acceptance and extensive use of post-tensioning did not begin until the late 1960's or early 1970's. The use of post-tensioning is especially suited to slab construction such as multi-level floor slabs in parking garages and buildings, or slabs-on-ground such as in residential construction. The post-tensioned slab-on-ground foundation for residential construction has become the system of choice for many builders in areas that have a relatively flat terrain. The primary reason for this is its economy and ease of installation.
Corrosion protection is provided for most of the tendon length by a coating of grease and a plastic sheath installed by the manufacturer. However, the plastic sheath must be removed at each terminal end for anchorage. This results in a length of bare tendon and an anchor assembly that are protected only by the concrete grout installed by the tensioner at the time of stressing. Corrosion problems typically result due to an inadequate thickness of concrete covering the terminal end of the tendon. The end of the tendon must be cut off a sufficient distance inside the face of the slab to prevent corrosion from starting and cracking the concrete grout seal. The minimum required clearance distance for prestressing steel from an exterior formed concrete surface is one inch. This minimum distance is critical in order to prevent contact of the steel with weathered, or carbonated, concrete. Carbonated concrete is concrete that has reacted with CO.sub.2 in the atmosphere over a period of time. This carbonation lowers the pH of the concrete to a level that is no longer non-corrosive to steel, and the steel begins to corrode. This is a condition that is beginning to affect numerous concrete structures where steel is present within the zone of carbonation. This condition is especially serious in post-tensioned structures since most pocket formers for post-tensioning anchorages make a pocket only 13/8 to 11/2-inches deep, and many tendons are not trimmed a sufficient distance inside the pocket.
Although post-tensioned structures are designed to be maintenance free for decades, and by all Codes the terminal end anchorages are supposed to be permanently protected against corrosion, it became apparent in the early 1980's that structures installed in corrosive environments were failing after only a few years due to failure of the grout seal and severe corrosion at the terminal end anchorages. Subsequently, coated tendons and encapsulating anchors were developed and are now used for corrosive environments. However, residential construction has continued with very little change from the methods used in the 1960's and 1970's. It has now become apparent that premature failure of concrete grout seals is causing failure of terminal and anchorages in all types of structures, and specifically in large numbers of single and multi-family residential structures. Most owners of post-tensioned structures are unaware of the type of reinforcing they have, and are not knowledgeable about inspection or maintenance of the terminal ends of the post-tensioned tendons.
Only recently has a tool become available for easily cutting the excess tendon length more than about 1/4-inch inside the pocket. This tool is a hydraulic shear device that fits into a standard pocket to cut the tendon and provide for installation of an adequate grout cover. The need for cutting the tendon a sufficient distance inside the face of the structure has been known for decades, but gas torch cutting has been banned in most areas due to the potential for heat damage in the anchorage zone, and the recently available hydraulic shear device is exorbitantly expensive to purchase and maintain.
The present invention is directed at the large numbers of tendons being installed in a normal service environment, such as in residential structures, where an economical means for properly trimming excess tendon length is needed, in order to permanently protect the anchorages from corrosion.